1. Field of the Invention
The invention relates to a method for forming a thin film resistor, and more particularly, to a method for forming a thin film resistor integrated with a metal gate process.
2. Description of the Prior Art
In the field of semiconductor fabrication, polysilicon material has been conventionally used to form the gate of metal-Oxide-Semiconductor Field-Effect (MOSFET) transistors. However, the polysilicon material possesses some drawbacks: the resistance of the polysilicon gate is higher than most of metal materials, thus the conductivity rate of the polysilicon gate is lower than metal wire. To compensate for this disadvantage, the polysilicon gate usually undergoes a silicide process for simultaneously reducing contact resistance and parasitic resistance (Rp), thus the conductivity rate of the polysilicon gate is improved to an acceptable range.
Secondly, the polysilicon gate also faces the boron penetration effect and an unavoidable depletion effect that deteriorate the performance of the device. Therefore the work function metal material approach which needs no ion implantation and thus avoids the boron penetration effect and the depletion effect ion effect is used to replace the polysilicon in gate formation.
It is noteworthy that the polysilicon is used not only to form the gate, but also to form passive devices of the mixed-mode integrated circuit devices, such as the capacitor, or the thin film resistor, etc. It is also well-known that the resistance of the thin film resistor made of polysilicon can be adjusted by modifying factors such as temperature or pressure in the polysilicon deposition process, or by adjusting the area, the thickness, or the concentration of implanted dopants to the polysilicon layer.
Nevertheless, in the trend to replace the polysilicon gate with the metal gate, those integratedly-formed passive devices which used to be made of polysilicon are also replaced with metal materials. Similar with the formation process of the active devices, passive devices such as thin film resistor is fabricated by integrating formations of the metal layer and the dielectric layer, the photolithography process, and the etching process. It is conceivable that the integration of metal thin film resistor process and the metal gate process, particularly the gate-first process, is more complicated while the thickness and composition controls for the materials in use are more difficult.
Therefore how to succeed in the integrating the metal gate and the thin film resistor without increasing process complexity and cost in such strict requirements has become an important study in the field.